For many new developments it has been found desirable to have electronic circuit packages not only small in area, but also very thin. For example, electronic circuits are being built into plastic credit cards which have area dimensions of about 2.12 by 3.37 inches and a thickness of about 0.04 inches. In addition, there has been found a need for a credit card size electronic circuit which includes an acoustic transducer for providing a sound to be sent over a telephone. Such acoustic transducers must not only be small and thin, but must also be capable of providing sound pressure levels of about 20 dynes per square centimeter for a minimum of -9 dBm electrical signal at telephone set line terminals.
Although there are miniature dynamic loudspeakers that use a moving coil and magnet structure, they are more than five times the thickness of a credit card. Thin piezoelectric ceramic diaphragm transducers are available in thickness of 0.02 inches. However, the ceramic material is brittle and subject to fracture in the event that the credit card is bent or sat upon. Electrostatic loudspeakers can be made in thin form. However, they require relatively large drive voltage amplitudes that are impractical with the limited battery power available in a credit card size circuit.
Piezoelectric plastic films, such as polarized polyvinylidene fluoride, has been used as the diaphragm and transducer element of an acoustic transducer. Such piezoelectric plastic film exhibits a transverse piezoelectric effect; i.e., when an electric field is applied perpendicularly to the film, a strain occurs in the plane of the film. Since a flat diaphragm of a piezoelectric plastic film cannot efficiently generate motion perpendicularly to the film diaphragm, cylindrical or spherical shaped films have been employed to translate transverse motion into linear motion normal to the film. Such dome-shaped thin films are generally achieved by applying back pressure with a compliant plastic foam material to maintain the shape. However, the foam introduces damping and stiffness to the motion of the film diaphragm and thereby serves to limit acoustic output. To overcome this problem there has been developed a design in which two circular, flat diaphragms are mounted with their peripheries clamped in spaced relation and the centers of the films being secured together so that each film is in the form of a cone. This design is shown in the U.S. patents of Preston V. Murphy, U.S. Pat. No. 4,295,010, issued Oct. 13, 1981 entitled PLURAL PIEZOELECTRIC POLYMER FILM ACOUSTIC TRANSDUCER, and U.S. Pat. No. 4,469,920, issued Sept. 4, 1984, entitled PIEZOELECTRIC FILM DEVICE FOR CONVERSION BETWEEN DIGITAL ELECTRIC SIGNALS AND ANALOG ACOUSTIC SIGNALS. However, it has been found that this design has a problem in that the thin film tends to wrinkle which results in low acoustic output and distortion.